I am no fan of excessively shiny, glassy, or delicious-looking dashboard components or graphs, but that effect can serve a purpose, as explained by the design principle of perceived affordance.
Affordance is more general: “a quality of an object, or an environment, that allows an individual to perform an action”. Don Norman, author of The Design of Everyday Things, elaborates on perceived affordance:
…Because I can click anytime I want, it is wrong to argue whether a graphical object on the screen “affords clicking.” It does. The real question is about the perceived affordance: Does the user perceive that clicking on that location is a meaningful, useful action to perform?
More from Universal Principles of Design, pg. 20:
Images of common physical objects and environments can enhance the usability of a design. For instance, a drawing of a three-dimensional button on a computer screen leverages our knowledge of the physical characteristics of buttons and, therefore, appears to afford pressing. The popular “desktop” metaphor used by computer operating systems is based on this idea – images of common items like trash cans and folders leverage our knowledge or how those items function in the real world and, thus, suggest their function in the software environment.
(They’re both buttons.)
In other words, is it obvious from the design of your application what can be clicked and what can’t? Giving your entire application the same flatness when certain components are intended to be clicked can cause areas of importance to be overlooked by the user. Similarly, it is misleading to make pie and bar charts look like lollipops and old-fashioned stick candy when they are not interactive.
I don’t know how much weight I would give this – because it’s subjective – but another design principle that could be relevant is the aesthetic-usability effect.
Advances in our understanding of emotion and affect have implications for the science of design. Affect changes the operating parameters of cognition: positive affect enhances creative, breadth-first thinking whereas negative affect focuses cognition, enhancing depth-first processing and minimizing distractions. Therefore, it is essential that products designed for use under stress follow good human-centered design, for stress makes people less able to cope with difficulties and less flexible in their approach to problem solving. Positive affect makes people more tolerant of minor difficulties and more flexible and creative in finding solutions. Products designed for more relaxed, pleasant occasions can enhance their usability through pleasant, aesthetic design. Aesthetics matter: attractive things work better.
Frankly, I don’t find these kinds of things attractive.